Display with observer tracking

ABSTRACT

The invention relates to a display, in particular an autostereoscopic or holographic display, for representing preferably three-dimensional information, wherein the stereo views or the reconstructions of the holographically encoded objects can be tracked to the movements of the associated eyes of one or more observers in a finely stepped manner within a plurality of zones of the movement region. In this case, the zones are selected by the activation of switchable polarization gratings.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of PCT/EP2013/051423, filed on Jan.25, 2013, which claims priority to German Application No. 10 2012201166.8, filed Jan. 26, 2012, and German Application No. 10 201205479.7, filed Jun. 22, 2012, the entire contents of each of which areincorporated fully herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a display, in particular an autostereoscopic orholographic display, for representing preferably three-dimensionalinformation, wherein the stereo views or the reconstructions of theholographically encoded objects can be tracked to the movements of theassociated eyes of one or more observers.

Displays for displaying three-dimensional information are known in amultiplicity of embodiments. Besides embodiments which requireadditional aids such as shutter or polarization spectacles for viewingthe spatial scene, autostereoscopic displays do not require such aids.Without additional measures, however, in the latter displays the 3-Dscene can be viewed only in a narrow spatial region, the so-calledobserver region. In order that the 3-D scene can also be seenconveniently in a large horizontal angular range, devices have beendeveloped which allow said observer region to be tracked to the eyes ofthe observer. Such a system has been described e.g. by the applicant inthe patent specification DE 103 39 076 B4. The applicant has likewisedescribed a series of holographic display devices, e.g. in EP 1 563 346B1 or EP 1 792 234 B1, wherein a 3-D scene can be perceived in anarrowly delimited observer region as a spatial reconstruction of theintensity distribution by means of holographic diffraction patternsbeing encoded in a spatial light modulator. Here as well it is expedientfor the observer region to be tracked to the eye movements of one ormore observers. For this purpose, such devices have a recognition systemthat determines the positions of the eyes of the observers and forwardsthe data to a system controller. The recognition systems often operatein a camera-based manner, wherein the eye positions are determined bymeans of image processing algorithms. In accordance with the respectiveeye positions, the system controller controls mechanical or electroniclight deflection means such that the center of the observer regionassigned to the respective eye position as far as possible correspondsto the respective eye position to the greatest possible extent. Ifdesired or necessary, at the same time the image content or thereconstruction of the 3-D scene can be adapted to the new eye position.

In such a display, an illumination unit generates light that iscollimated to the greatest possible extent with the required wavelengthspectrum with which the spatial light modulator into which thestereoscopic views are written or the holographic information is encodedis illuminated. In the case of holographic displays, the light mustadditionally be capable of interference at least in a region requiredfor encoding a pixel of the 3-D scene, i.e. said light must besufficiently coherent. The light deflection means can be situatedupstream and/or downstream of the spatial light modulator.

The illumination unit can also be configured in a controllable mannersuch that it can direct light in predefinable spatial directions. Forthis purpose, for example, displaceable illumination columns can besituated in direct proximity to the focal plane upstream of acylindrical lens array. The illumination columns can be controllablyselected for example from an active light source matrix, which can beembodied e.g. as an OLED matrix. It is also possible to use a planarlight source, in front of which is arranged an array of slit diaphragmsthat are variable in drivable manner in terms of their horizontalposition. Such a diaphragm array can be configured as an LCD matrix, forexample. In this case, each column forms a secondary light source whichilluminates that cylindrical lens of the cylindrical lens array which isassigned to it. The horizontal position of the illumination columns withrespect to the center line of the assigned cylindrical lens determinesthe horizontal angle—emitted by the respective cylindrical lens—of thepartial beam that is collimated to the greatest possible extent. In thiscase, a plurality of illumination columns can also be activatedsimultaneously in order to increase the emitted angular range and thusthe size of the assigned observer region. Furthermore, the horizontaldeviation of the position of the illumination columns with respect tothe center line of the assigned cylindrical lens can vary over the areaof the cylindrical lens array in order e.g. to realize an additionalfield lens function and thus to adapt the horizontal diameter of theassigned observer region to the observer distance. The cylindrical lensarray can have a diaphragm array that avoids crosstalk between theillumination columns assigned to a cylindrical lens and neighboringcylindrical lenses. Undesired secondary observer regions can thus beavoided.

The number of illumination columns assigned to a cylindrical lensdetermines the number of possible horizontal deflection angles. Thisnumber cannot be increased arbitrarily, since the primary and/orsecondary light sources have technically dictated minimum dimensions.Moreover, the required luminance increases, the smaller the dimensionsbecome. For a large horizontal movement region in which a plurality ofobservers can also be situated, however, a large number of finelygradated light deflection angles are required for observer tracking.Therefore, a series of additional measures have already been proposedfor increasing said movement region and thus the number of possibledeflection angles. By way of example, in the U.S. Pat. No. 7,791,813 B2,arrays of electrically controllable electrowetting cells were to be usedfor beam deflection. However, such arrays are complicated to produce andhave a restricted aperture on account of the cell height. Mechanicaldeflection means such as deflection mirrors or rotatable prisms, whichwere likewise proposed, are sluggish and require a large structuralvolume.

Therefore, the present invention is based on the object of specifyingand developing a display of the type mentioned in the introduction whichovercomes the problems mentioned above. In particular, the intention isto enable the observer tracking for one or more observers in a finelystepped manner in a plurality of movement zones, which preferablyoverlap, without mechanical light deflection means.

SUMMARY OF THE INVENTION

The object is achieved according to the invention by means of theteaching of patent claim 1. Further advantageous configurations anddevelopments of the invention are evident from the dependent claims.

According to the invention, a display, in particular an autostereoscopicor holographic display, wherein image views or reconstructions ofholographically encoded objects can be tracked to the movements ofassociated eyes of at least one observer, has an illumination unitcomprising light sources for generating light that is collimated to thegreatest possible extent with a predefinable emission characteristic, atleast one polarization grating stack having a stack of opticalcomponents comprising at least two switchable or controllablepolarization gratings each comprising a grating structure that can beswitched on for light deflection, a spatial light modulator formodulating the light of the illumination unit for displaying imageinformation or for reconstructing encoded hologram information, arecognition system for determining positions of the associated eyes ofthe at least one observer, and a system controller for driving andsynchronizing the switchable and controllable elements, wherein,depending on the current position of the eyes of a observer, theillumination unit and the polarization grating stack are drivable by thesystem controller in such a way that the light of the illumination unitis deflectable in the direction of the eyes of the observer.

In this case, the emission characteristic of the illumination unit isdetermined by the type of display. For holographic displays, use ispreferably made of narrowband light sources having good coherenceproperties, such as are constituted by laser light sources, for example.For autostereoscopic displays, it is possible to use inorganic ororganic luminescence diodes, for example, which have a wider wavelengthspectrum.

The polarization gratings of the polarization grating stack are drivenby the system controller such that a respective grating is active anddirects the light in the predefined direction in order thus to generatea zone of the movement region. The switchable polarization gratings canbe configured as switchable liquid crystal polarization gratings, forexample. For this purpose, the liquid crystal alignment layer has aperiodically recurring structure for locally changing the liquid crystalorientation that determines the polarization of the light passingthrough. The shorter this period, the greater the diffraction angle andthus the light deflection angle for the predefined wavelength of thelight. By applying a voltage to electrodes which are assigned to theliquid crystal layer, it is possible for the alignment of the liquidcrystal molecules that is brought about by the liquid crystal alignmentlayer to be canceled and thus for the polarization grating to beswitched off. The light then passes through this grating without beingdeflected.

The grating period can be varied locally in order, for example,additionally to enable a field lens function.

The light sources of the illumination unit can be arranged in an arrayof primary light sources as a light source matrix, which are switchableor controllable individually or in columns. Said light sources can beLEDs, OLEDs or laser diodes, for example.

The light sources can be controlled separately in terms of theirbrightness by the system controller, in order e.g. to compensate forlocally different transparencies in the beam path or to improve thecontrast when displaying the image information.

In order to form a color display, different light sources are preferablyused, which differ in terms of their emitted wavelength spectrum, inparticular the focus wavelength, and which are separately operable. Theindividual colors can thus be generated in time division multiplex, i.e.in temporal succession. For this purpose, the system controllersynchronizes the driving of the respective color with the writing of theassociated color information into the spatial light modulator.

The different image contents or encoding information for the right andleft eyes can likewise be written time-sequentially in a synchronizedmanner by the system controller. In this case, the required deflectiondirections for generating the associated observer region arerespectively set for each eye. It is also possible, in the case of aplurality of observers, to write a dedicated view or encodinginformation item for each observer eye. Color and view changes can besuitably combined, such that as far as possible no disturbances, inparticular disturbing flickering, become visible to the observers.

A controllable diaphragm array, which can be configured for example asan array of controllable slit diaphragms, can be arranged upstream ofthe polarization grating stack. This array can be integrated in theillumination unit.

The diaphragm array is preferably illuminated by a planar light source.

Such a diaphragm array can consist of a matrix of individually drivableliquid crystal cells, for example. Said cells can be arranged incolumns. However, a two-dimensional matrix arrangement of controllablecells can also be used, wherein the correct columns are selected by thedriving of the associated cells by the system controller and thus formsecondary columnar light sources.

With the use of a light source matrix or an array of slit diaphragms,the control of the light sources or the position of the slit diaphragmscan be set such that this compensates for the wavelength dependence ofthe deflection angles of the switchable polarization gratings of thepolarization grating stack. In the case of time-sequential illuminationwith red, green and blue light, for example, the position of the slitdiaphragms is set for each color in such a way that for all colors lightis directed to the same detected observer position.

The illumination unit can contain a cylindrical lens array for thepurpose of collimation. In this case, the cylindrical lenses are alignedalong the illumination columns. Preferably, the illumination columns,which are correspondingly embodied as primary or secondary lightsources, are situated to the greatest possible extent in the focal planeof the cylindrical lenses. The cylindrical lenses can also be embodiedas gradient-index lenses. A multi-stage construction with a plurality ofimaging surfaces arranged one behind another can also be employed,wherein at least one intermediate imaging can also be effected. As aresult of the displacement of the illumination columns by the systemcontroller transversely with respect to the cylindrical lenses, finelygradated tracking of the light of the illumination unit in the directionof the eyes of the observer can be effected.

In this case, the columns can be driven such that the tracking anglevaries over the area of the cylindrical lens array, in order to obtain afield lens function and/or to achieve an adaptation of the width of theobserver region assigned to the respective eye to the observer distance,in order that from all positions of the movement region a 3-D view canbe perceived as far as possible without any disturbances.

The cylindrical lens array can itself already contain a field lensfunction, which for example forms a observer region at the preferredobserver position. Such a preferred observer position is situated e.g.in the center of the horizontal movement region at the average movementdistance.

The illumination unit can set the circular polarization direction of thelight that is required for the downstream polarization grating stack.

It is also possible to arrange for that purpose a separate means thatinfluences the polarization of the light upstream of the polarizationgrating stack. Such a means can contain a birefringent retardationlayer, for example, which is configured as a quarter-wave plate andconverts linear light, emitted purely by way of example by theillumination unit, into circularly polarized light.

An additionally fixed or variable field lens can be contained in thebeam path between the illumination unit and the observer in order topredefine the width of the observer region assigned to the respectiveeye at the predefined location of the respective observer eye or to setit depending on said location. The field lens can also be part of theillumination unit, situated between the latter and the polarizationgrating stack, arranged downstream of the polarization grating stack orsituated between spatial light modulator and observer.

The polarization grating stack is preferably arranged in the light pathbetween the illumination unit and the spatial light modulator. However,it is also possible to arrange the polarization grating stack in thelight path downstream of the spatial light modulator.

The polarization grating stack can preferably contain as opticalcomponent at least one additional switchable Or controllablebirefringent retardation layer, preferably configured as a switchable orcontrollable half-wave plate. In the switched-off state the circularlypolarized light that radiates through said layer maintains its directionof rotation. In the switched-on state, the direction of rotation of thecircularly polarized light is changed and thus corresponds to thedirection of rotation of a switched-on polarization grating of thepolarization grating stack for the same direction of rotation of theinput polarization. What can be achieved with this birefringentretardation layer is that at the output of the polarization gratingstack, for the same input direction of rotation, the same outputdirection of rotation is always achieved, irrespective of whether or nota grating is switched on, since, with a polarization grating switchedoff, the light is transmitted without being deflected and without achange in the direction of rotation of the circular polarization. If thebirefringent retardation layer is configured in a controllable fashion,for example as a controllable liquid crystal layer, it is possible tocompensate for dispersion effects or changes in the effective opticalpath length in the case of oblique beam passage. For this purpose, theretardation layer is to be synchronized with the other active componentsby means of the system controller.

An additional retardation layer can be situated downstream of thepolarization grating stack in order to convert circularly polarizedlight into linearly polarized light. That is advantageous for example ifthe downstream assembly, for example the spatial light modulator,requires linearly polarized light for its operation. Such a retardationlayer can be configured as a quarter-wave plate, for example. For colordisplays, said retardation layer can advantageously be configured in anachromatic or apochromatic fashion. It is also possible for saidbirefringent retardation layer to be configured in a controllablefashion, for example as a controllable liquid crystal layer, in order tocompensate for dispersion effects or changes in the effective opticalpath length in the case of oblique beam passage. For this purpose, theretardation layer is to be synchronized with the other active componentsby means of the system controller.

A polarization filter can advantageously be arranged in the light pathdownstream of said retardation layer, said polarization filtersuppressing linearly polarized light of the zeroth order of diffractionof a switched-on polarization grating of the polarization grating stack,that is to say light which passes through the polarization gratingwithout diffraction. This light is linearly polarized and isperpendicular to the light which leaves the retardation layer afterdeflection by a switched-on polarization grating of the polarizationgrating stack. Said polarization filter can also be part of a downstreamspatial light modulator, for example if the latter requires linearlypolarized light for its manner of operation.

Light sources that are controllable in terms of their direction have ahigh power demand, particularly if they operate with controllable slitdiaphragms. It is therefore particularly advantageous, instead of lightsources that are controllable in terms of their direction or in order tosupport said light sources, to arrange a controllable deflection gratinghaving a variable grating period upstream of, downstream of or in thepolarization grating stack, with which grating period a finely steppedadditional light deflection can be carried out. Such gratings areadvantageously embodied as polarization gratings having a variablegrating period. Such controllable polarization gratings can beconfigured for example as electrically controllable liquid crystalcells. In this case, the polarization of the light passing through isinfluenced locally by a voltage pattern being applied to a finelydimensioned electrode structure, wherein the magnitude of the appliedvoltage determines the alignment state of the liquid crystal molecules.Like the switchable polarization gratings of the polarization gratingstack, these gratings diffract circularly polarized light only in oneorder of diffraction. The diffraction angle can be set by means of theperiod of the voltage profile. The diffraction direction is determinedby the local voltage profile within a period. Preferably, the voltageprofile is saw tooth-shaped, wherein the direction of the pulse rampdetermines the deflection direction. Since the variable polarizationgrating need have only a small deflection angle range, a large anglerange is achieved by the zone division with the switchable polarizationgratings, and the requirements made of the fineness of the electrodestructure can be kept in a realizable range.

The period of the voltage profile to be applied to the electrodestructure can be varied over the area of the grating in order toadditionally obtain or support a field lens function. It is particularlyadvantageous that the focus of said field lens can be changed solely bythe electronic driving by means of the system controller.

Such a controllable variable grating can also advantageously be used tocompensate for the wavelength dependence of the deflection angles of theswitchable polarization gratings of the polarization grating stack.

The illumination unit can advantageously also be configured such that ithas controllably different light emergence angles. This can be, forexample, an array of directional light sources which have differentemission directions either in a spatially alternating manner or in atemporally switchable manner.

At least one optical component in a polarization grating stack used canbe embodied as segmented in one or two directions, wherein theindividual segments can be separately switched or controlled. Thus,different deflection angles can be realized in a manner dependent on thepassage location of the light through the polarization grating stack, inorder for example to form or support an additional field lens functionin order to better track light to a observer for example in the case ofan extensive display.

The segmentation can for example also be effected in the form ofconcentric circular or elliptic rings.

The grating structure of at least one polarization grating in thepolarization grating stack can also be arranged in a manner rotated withrespect to other polarization gratings in order to enable atwo-dimensional deflection. In this case, the individual polarizationgratings can differ in terms of their grating constant in order toenable different deflection angles in the directions rotated withrespect to one another. It is particularly advantageous in this case ifthe grating structures of the polarization gratings are alignedperpendicularly to one another.

Polarization gratings rotated with respect to one other in terms oftheir grating structure can also be arranged in separate polarizationgrating stacks in order for example to arrange further components suchas controllable deflection gratings having a variable grating periodand/or light modulators and/or field lenses between them. In thisregard, in each case a separate controllable deflection grating having avariable grating period for a finely stepped deflection of therespective deflection direction can be arranged downstream of theassociated polarization grating stack.

By changing the voltage at a controllable polarization grating, it ispossible to influence the deflection effectiveness and thus thatproportion of the light which is deflected in the +1^(st) or −1^(st)order of diffraction depending on the circular polarization direction.Besides control of the intensity of the light sources and/or themodulation intensity of the light modulator, it is thus possible toreduce intensity fluctuations, for example, such as can occur forexample as a result of different diffraction effectivenesses forindividual polarization gratings of the polarization grating stack orfor different spectral distributions of the light sources of theillumination device or further angle-dependent optical losses of theoptical system.

Conventional polarization gratings have the property that they alter thepolarization direction of the light passing through. By way of example,if left circularly polarized light is incident on such a polarizationgrating, then the light deflected by said grating emerges again in rightcircularly polarized fashion. Right circularly polarized light deflectedby such a grating correspondingly emerges in left circularly polarizedfashion, wherein the signs of the associated first order of diffractiondiffer. Since the deflection direction, i.e. the sign of the associatedfirst order of diffraction for downstream polarization gratings in astack of polarization gratings is also influenced by the circularpolarization direction incident on said polarization gratings, it isnecessary to take account of or set the possible polarization changes inthe arrangement of the polarization gratings in the stack, as alreadydescribed. This is advantageously done by means of switchable orcontrollable retardation layers between the polarization gratings inorder to adapt the direction of rotation of the circularly polarizedlight to the desired deflection direction, that is to say to the +1^(st)or −1^(st) order of diffraction.

A publication “Twisted nematic liquid crystal polarization grating withthe handedness conservation of a circularly polarized state” by Honmaand Nose, Optics Express Vol. 20, pages 18449-18458, 2012, describes aspecific type of polarization grating in which the deflected lightemerging from the polarization grating has the same circularpolarization state as the incident light. This type of polarizationgrating is based on a liquid crystal structure having a periodic twist.The setting of such a twist is achieved by means of a periodic surfaceorientation of the liquid crystal molecules in the liquid crystal layerwhich has an opposite direction of rotation on both substrates. What isdisadvantageous about this type of polarization grating is thatrelatively thick liquid crystal layers are required, such that smallgrating periods and short switching times can be realized only withdifficulty. Therefore, the majority of the exemplary embodimentsmentioned below relate to conventional polarization gratings. Generally,the invention is also applicable to the type of grating described in thepublication by Honma and Nose. A polarization grating stack can alsocontain gratings of both types in mixed form. In these gratings, too,the desired deflection direction can be selected by means of thedirection of rotation of the circularly polarized light entering therespective polarization grating.

There are, then, various possibilities of advantageously configuring anddeveloping the teaching of the present invention and/or of combining theabove-described embodiments—insofar as possible—with one another. Inthis respect, reference should be made firstly to the patent claimssubordinate to patent claim 1 and secondly to the following explanationof the preferred exemplary embodiments of the invention with referenceto the drawings. Generally preferred configurations and developments ofthe teaching are also explained in conjunction with the explanation ofthe preferred exemplary embodiments of the invention with reference tothe drawing. In the drawing, in each case in a schematic illustration:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a shows an explanation of the symbols used in the drawings for thepolarization directions,

FIG. 1b shows a first configuration variant with displaceable lightsource columns in a controllable slit diaphragm array 120 in which asecond polarization grating 340 of a polarization grating stack 300 isswitched on,

FIG. 1c shows a first configuration variant with displaceable lightsource columns in a controllable slit diaphragm array 120 in which afirst polarization grating 310 of a polarization grating stack 300 isswitched on,

FIG. 1d shows a first configuration variant with displaceable lightsource columns in a controllable slit diaphragm array 120 in which aswitchable half-wave plate 320 of a polarization grating stack 300 isswitched on,

FIG. 1e shows a first configuration variant with displaceable lightsource columns in a controllable slit diaphragm array 120 in which twopolarization gratings 310, 340 and a switchable half-wave plate 320 of apolarization grating stack 300 are switched on,

FIG. 2 shows a second configuration variant with a controllable variablepolarization grating 200 and a field lens 600 in which a secondpolarization grating 340 of a polarization grating stack 300 is witchedon,

FIGS. 3a and 3b show a third configuration variant, wherein theillumination unit 100 additionally has controllably different lightemergence angles,

FIG. 4 shows a fourth configuration variant with an additional fieldlens 600,

FIG. 5 shows a fifth configuration variant, in which the diaphragms inthe slit diaphragm array 120 are set such that a field lens function isrealized with the cylindrical lens array 150,

FIG. 6 shows a sixth configuration variant, which additionally containsa controllable deflection grating having a variable grating period 700,

FIG. 7 shows a seventh configuration variant similar to FIG. 5, but witha spatial subdivision of the polarization gratings 310, 340 and thehalf-wave plate 320 in the polarization grating stack 300,

FIG. 8 shows an eighth configuration variant similar to FIG. 5, in whichthe polarization grating stack 300 contains an additional switchablehalf-wave plate 350 and an additional polarization grating 360 for atwo-dimensional light deflection,

FIG. 9 shows a ninth configuration variant similar to FIG. 6, whichcontains an additional switchable or controllable polarization gratingstack 305 and an additional controllable deflection grating having avariable grating period 705 for a two-dimensional light deflection, and

FIG. 10 shows a tenth configuration variant similar to claim 6, in whichthe polarization grating stack 300 has polarization gratings 380, 390which are based on a twisted structure and which do not change thepolarization direction in the case of a light deflection.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows the symbols—used in FIGS. 1 to 6—of the polarizationdirection leaving the respective illustrated optical element for thelight passing through. The symbols are situated above the relevantoptical elements in the drawings. Right and left circular polarizationand vertical and horizontal linear polarization are illustrated.

A first configuration variant of the invention is illustrated purelyschematically in FIGS. 1b to 1e . An illumination unit 100 that iscollimated to the greatest possible extent and is controllable in theemission direction contains a light source matrix 110, a controllableslit diaphragm array 120, which can be embodied as a controllable liquidcrystal matrix, a linear polarization filter 130, which canadvantageously be configured as an output polarizer of the liquidcrystal matrix, a birefringent retardation layer 140, for generating therequired circular polarization from the linear polarization, which canadvantageously be configured as a quarter-wave plate, and a cylindricallens array 150 for collimating the light stripes emerging from the slitdiaphragms. The birefringent retardation layer 140 or the birefringentretardation layer 140 and the linear polarization filter 130 can also besituated in the light path downstream of the cylindrical lens array 150.Purely by way of example, left circularly polarized collimated lightleaves the cylindrical lens array 150 and impinges on the polarizationgrating stack 300, which contains here purely by way of example a firstswitchable polarization grating 310, a switchable birefringentretardation layer 320, which can advantageously be configured as aswitchable half-wave plate, and a second switchable polarization grating340. The polarization grating stack 300 is followed here by abirefringent retardation layer 410, which can advantageously beconfigured as a quarter-wave plate, for generating linearly polarizedlight from the right circularly polarized light leaving the polarizationgrating stack. In a downstream linear polarization filter 420, thislight is converted into horizontally polarized light. At the same time,vertically polarized light that arises in the zeroth order ofdiffraction of a switched-on polarization grating is blocked. Thepolarizer 420 is followed by a spatial light modulator 510, which isfollowed here purely by way of example by a polarization filter 530serving as an analyzer. The polarizer 420, as illustrated in FIG. 2, canalso serve as an analyzer for the spatial light modulator.

In FIG. 1b , the second polarization grating 340 is switched on, i.e.this diffraction grating is active and directs the light into its zone(not illustrated here) of the movement region of the observers. At thisgrating, no voltage that destroys the grating-shaped alignment of theliquid crystal molecules is present at the electrodes. Accordingly,right circularly polarized light leaves the second switchablepolarization grating 340. The first switchable polarization grating 310and the switchable half-wave plate 320 do not influence the polarizationstate.

In FIG. 1c , the first switchable polarization grating 310 is activatedand generates that zone of the movement region of the observers which isassigned to it. Here right circular light leaves this polarizationgrating 310 and its polarization direction is not altered in thedownstream non-activated switchable birefringent retardation layer 320and the second non-activated switchable polarization grating 340.

In FIG. 1d , neither of the two switchable polarization gratings 310 and340 is activated. The light passes through the polarization gratingstack 300 without being deflected and forms a third zone of the movementregion for the observers. The activated birefringent retardation layer320 here provides for the necessary rotation of the direction ofrotation of the polarization from left circular to right circular, inorder that the correct linear polarization direction arrives at thepolarization filter 420.

In FIG. 1e , both polarization gratings 310 and 340 are activated. Acombination of the light deflection of both polarization gratings 310and 340 can thus be used for observer tracking. In this case, too, theactivated birefringent retardation layer 320 provides for the rotationof the direction of rotation of the polarization, in order that thecorrect linear polarization direction arrives at the polarization filter420.

The controllable birefringent retardation layer 320 is activatedwhenever an even number of polarization gratings is activated. Thecontrollable birefringent retardation layer 320 is not activated when anodd number of polarization gratings is activated. As a result, anidentical direction of rotation of the circular polarization downstreamof the polarization grating stack 300 is always obtained.

FIG. 2 shows a second exemplary embodiment. Here, an illumination unit100 generates purely by way of example collimated left circularlypolarized light. The latter illuminates a downstream controllablepolarization grating 200, the grating period of which is used to set thebeam direction in the activated zone of the downstream polarizationgrating stack 300, which here likewise purely by way of example containsa first switchable polarization grating 310, a switchable birefringentretardation layer 320 and a second switchable polarization grating 340.The polarization filter 520 downstream of the birefringent retardationlayer 410 is assigned here to the spatial light modulator 510, whichlikewise has a polarization filter 530 as an analyzer. Downstream in thebeam path, a field lens 600 is provided here purely by way of example,said field lens advantageously being configured as a flat Fresnel lens.

FIGS. 3a and 3b show a further exemplary embodiment similar to that inFIG. 2. In contrast to FIG. 2, it contains an illumination unit 100having controllably different light emergence angles. In this case, FIG.3a shows schematically, in a manner indicated by arrows, onecontrollably set light emergence angle and FIG. 3b shows anothercontrollably set light emergence angle of the illumination unit 100.

In this example, the set light emergence angles both lie in the plane ofthe drawing. In general, an illumination unit 100 can also haveadditional settable light emergence angles for example perpendicular tothe plane of the drawing.

FIG. 4 shows a further exemplary embodiment. In contrast to the firstexemplary embodiment according to FIGS. 1b to 1e , an additional fieldlens 600 is provided here as in the second exemplary embodimentaccording to FIG. 2. In this case, FIG. 4 schematically depicts lightbeams which emerge from the slit diaphragm array 120 and are focused bythe field lens 600. Both polarization gratings 310 and 340 areillustrated as not activated, as in FIG. 1d , with the result that thelight passes through the polarization grating stack 300 without beingdeflected.

FIG. 5 shows an exemplary embodiment corresponding to the exemplaryembodiment shown in FIGS. 1b to 1e . Here, however, the positions of thediaphragms in the slit diaphragm array 120 are set such that, incombination with the cylindrical lens array 150, the light emerging fromdifferent slit diaphragms is focused onto a common position. In thiscase, therefore, the cylindrical lens array also contains the field lensfunction, such that, in contrast to the exemplary embodiments accordingto FIGS. 2 to 4, an additional field lens can be dispensed with. Here aswell, both polarization gratings 310 and 340 are illustrated as notactivated, as in FIG. 1d , such that here as well the light passesthrough the polarization grating stack 300 without being deflected.

FIG. 6 shows a configuration variant similar to FIG. 2. In additionthereto, it contains a controllable deflection grating having a variablegrating period 700. In this example, the grating is arranged downstreamof the polarization grating stack 300, the light modulator 510 and thefield lens 600. The illustration shows that the second polarizationgrating 340 of the polarization grating stack 300 is activatedanalogously to FIG. 1b . Such a variable deflection grating 700 enablesan additional finely stepped light deflection and/or compensation of thewavelength dependence of the light deflection in the polarizationgrating stack 300.

It is also possible to arrange the variable deflection grating upstreamof the polarization grating stack 300 or between two individualcomponents 310, 320, 340 of the polarization grating stack 300. Thefunction of the deflection grating can also be divided among a pluralityof components which can be situated at different locations in the beampath.

FIG. 7 shows a further configuration variant similar to FIG. 5. In thiscase, the polarization gratings 310 and 340 and the switchable half-waveplate 320 are subdivided into a plurality of separately switchablesegments. In the example shown, purely by way of example, each opticalcomponent (310, 320, 340) was subdivided into two segments (311, 312,321, 322, 341, 342). The polarization grating 310 is illustrated asactivated in the upper segment (312); the switchable half-wave plate 320is illustrated as activated in the lower segment. Downstream of thepolarization grating stack 300, the emerging polarization state is thesame in both segments, but the deflection angle differs in the upper andlower segments. This subdivision can be used for tracking a observer inthe case of an extensive display. The segmentation can also be effectedtwo-dimensionally or, for example, concentrically.

FIG. 8 shows a configuration variant similar to FIG. 5. In contrast toFIG. 5, the polarization grating stack contains an additional switchablehalf-wave plate 350 and an additional polarization grating 360. Theseelements are illustrated as activated in the figure. In the polarizationgrating 350, the grating structure is arranged in a manner rotated by 90degrees in comparison with the polarization gratings 310 and 340. As aresult, the deflection direction is also rotated by 90 degrees relativeto the deflection direction of the gratings 310 and 340. Such apolarization grating stack can be used for example in combination with atwo-dimensional arrangement of spherical lenses 155 and an array 125 ofsquare or round diaphragms which are controllable in two directions.Observer tracking both in the horizontal direction and in the verticaldirection thus becomes possible.

The grating structures of the two polarization gratings 310, 340 neednot be arranged orthogonally with respect to the grating structure ofthe polarization grating 350, rather they can be arranged such that thedeflection can be effected in two arbitrarily chosen directions.

Other illumination devices 100 that are controllable in two directionscan also be employed.

FIG. 9 shows a configuration variant similar to FIG. 6. However, itadditionally has a second switchable or controllable polarizationgrating stack 305, whose grating structure of the switchable orcontrollable polarization gratings 315, 345 illustrated is rotatedrelative to the grating structure of the two polarization gratings 310,340 in the polarization grating stack 300. This configuration containstwo controllable deflection gratings having a variable grating period700 and 705. In this case, purely by way of example, the additionalcontrollable deflection grating 705 having a variable grating period isassigned to the polarization grating stack 305, the diffractiondirection of said deflection grating being coordinated with thediffraction direction of the polarization grating stack 305. Thecontrollable deflection grating having a variable grating period 700 isassigned to the polarization grating 300 and is coordinated with thedeflection direction thereof. Purely by way of example, it is arrangedbetween the two polarization grating stacks 300, 305. In this case, thedeflection grating 700 serves for example for horizontal lightdeflection, and the grating 705 for vertical light deflection. Thevariable deflection gratings 700 and 705 enable an additional finelystepped light deflection and/or compensation of the wavelengthdependence of the light deflection in the polarization grating stacks300 and 305.

The configuration variants in FIGS. 1 to 9 relate to the use ofconventional polarization gratings. These polarization gratings have theproperty that they alter the polarization of the incident light, forexample from left circular to right circular.

FIG. 10 shows a configuration variant of the invention in which thepolarization grating stack 300 contains switchable or controllablepolarization gratings 370, 380 which are based on a periodically twistedstructure and in which the light passing through maintains the directionof rotation of its polarization. The configuration variant isconstructed in a manner similar to FIG. 6. However, the firstpolarization grating 370 of the polarization grating stack 300 isillustrated as activated, that is to say that the light passes throughthis polarization grating in a diffracted and thus deflected manner.When the light passes through this polarization grating 370, the lightmaintains its direction of rotation of the polarization in the same wayas when it passes through the second polarization grating 380, which isillustrated here as not activated, such that no further light deflectiontakes place therein.

If the intention is to use only one deflection direction, that is to saythat only one fixed input polarization is selected for each grating 370,380, it is possible to dispense with a switchable or controllableretardation layer between two successive switchable or controllablepolarization gratings 370, 380, as is illustrated purely by way ofexample here in FIG. 10. Since the polarization gratings 370, 380 usedhere do not change the direction of rotation of the polarization aslight passes through, advantageously in this configuration variant nopolarization rotating layers are required between the polarizationgratings 370, 380 if polarization gratings having an identical twist areused in the layer stack. In this example, the illumination unit 100generates right circularly polarized light. This right circularpolarization state is maintained during passage through bothpolarization gratings 370, 380. For a spatial light modulator 510 thatrequires linearly polarized light, as in FIG. 6 a birefringentretardation layer 410 can be included and undesired stray light of thezeroth order of diffraction can be suppressed by means of a linearpolarizer 520.

Finally, it should be pointed out very particularly that the exemplaryembodiments discussed above serve merely to describe the claimedteaching, but do not restrict the latter to the exemplary embodiments.In particular, the exemplary embodiments described above could—insofaras is possible—be combined with one another.

LIST OF REFERENCE SIGNS

-   100 Illumination unit-   110 Light source matrix-   120 Controllable slit diaphragm array-   125 Controllable diaphragm array-   130 Polarization filter-   140 Birefringent retardation layer-   150 Cylindrical lens array-   155 Lens array-   200 Variable controllable polarization grating-   300 Polarization grating stack-   305 Rotated polarization grating stack-   310 1^(st) switchable polarization grating-   311 1^(st) segment of the 1^(st) switchable polarization grating-   312 2^(nd) segment of the 1^(st) switchable polarization grating-   315 1^(st) switchable polarization grating in the rotated    polarization grating stack-   320 switchable birefringent retardation layer-   321 1^(st) segment of the switchable birefringent retardation layer-   322 2^(nd) segment of the switchable birefringent retardation layer-   325 Switchable birefringent retardation layer in the rotated    polarization grating stack-   335 Switchable birefringent retardation layer in the rotated    polarization grating stack-   340 2^(nd) switchable polarization grating-   341 1^(st) segment of the 2^(nd) switchable polarization grating-   342 2^(nd) segment of the 2^(nd) switchable polarization grating-   345 2^(nd) switchable polarization grating in the rotated    polarization grating stack-   350 Switchable birefringent retardation layer-   360 Rotated switchable polarization grating-   370 1^(st) switchable polarization grating maintaining the direction    of polarization-   380 2^(nd) switchable polarization grating maintaining the-   direction of polarization-   410 Birefringent retardation layer-   420 Linear polarization filter-   510 Spatial light modulator-   520 Linear polarization filter-   530 Linear polarization filter-   600 Field lens-   700 Controllable deflection grating-   705 Rotated controllable deflection grating

The invention claimed is:
 1. A display, in particular anautostereoscopic or holographic display, wherein image views orreconstructions of holographically encoded objects can be tracked tomovements of associated eyes of at least one observer with aneye-position recognition system, comprising: an illumination unit forgenerating light that is collimated to the greatest possible extent witha predefinable emission characteristic, the light of the illuminationunit having a predefinable circular polarization at least onepolarization grating stack having a stack of optical componentscomprising at least two switchable or controllable polarizationgratings, each comprising a grating structure that can be switched onfor light deflection, a spatial light modulator for modulating the lightof the illumination unit for displaying image information or forreconstructing encoded hologram information, and a system controller fordriving and synchronizing the switchable or controllable polarizationgratings, the illumination unit and the spatial light modulator,wherein, depending on the current position of the eyes of an observer,the illumination unit and the polarization grating stack are drivable bythe system controller in such a way that the light of the illuminationunit is deflectable in the direction of the eyes of the observer.
 2. Thedisplay as claimed in claim 1, wherein the polarization grating stackadditionally comprises as optical component at least one switchable orcontrollable birefringent retardation layer.
 3. The display as claimedin claim 1, wherein an additional birefringent retardation layer issituated downstream of the polarization grating stack in order toconvert circularly polarized light into linearly polarized light.
 4. Thedisplay as claimed in claim 3, wherein a polarization filter is situateddownstream of the additional birefringent retardation layer, saidpolarization filter suppressing linearly polarized light of the zerothorder of diffraction of a switched-on polarization grating.
 5. Thedisplay as claimed in claim 1, wherein a controllable deflection gratinghaving a variable grating period is arranged upstream or downstream ofor in the polarization grating stack.
 6. The display as claimed in claim1, wherein the illumination unit comprises controllably different lightemergence angles.
 7. The display as claimed in claim 1, wherein at leastone optical component in the polarization grating stack is embodied assegmented in one or two directions, wherein the individual segments areseparately switchable or controllable.
 8. The display as claimed inclaim 1, wherein the grating structure of at least one polarizationgrating in the polarization grating stack is arranged in a mannerrotated with respect to one another, or in that the display comprises asecond polarization grating stack, wherein the grating structure, withrespect to the grating structure of the first polarization grating stackis rotated by an angle with respect to one another.
 9. The display asclaimed in claim 1, wherein the polarization grating stack comprises atleast one switchable or controllable polarization grating in which thedirection of rotation of the light upon passing through the polarizationgrating is maintained.
 10. The display as claimed in claim 1, whereinthe polarization grating stack is arranged in the light path between theillumination unit and the spatial light modulator.
 11. The display asclaimed in claim 1, wherein the light sources of the illumination unitare arranged in an array of primary light sources as a light sourcematrix, which are switchable or controllable individually or in columns.12. The display as claimed in claim 1, wherein the light sources areLEDs or OLEDs.
 13. The display as claimed in claim 1, wherein the lightsources consist of a plurality of separately operable individual lightsources having different focus wavelengths of their emissioncharacteristic.
 14. The display as claimed in claim 1, wherein theillumination unit comprises a switchable or controllable array of slitdiaphragms.
 15. The display as claimed in claim 14, wherein the array ofslit diaphragms is embodied as an LCD diaphragm array.
 16. The displayas claimed in claim 1, wherein the illumination unit comprises acylindrical lens array for the purpose of collimation.
 17. The displayas claimed in claim 16, wherein the cylindrical lens array additionallycomprises a field lens function.
 18. The display as claimed in claim 1,wherein light of the illumination unit has a predefinable circularpolarization.
 19. The display as claimed in claim 1, wherein anadditionally fixed or variable field lens is contained in the beam pathbetween the illumination unit and the observer.